Smartwatch comfort is a fundamental factor that significantly influences the user experience and provides crucial guidance for the evolution of wearable technology. However, there is currently a lack of a comprehensive theoretical model to describe the dimensions of smartwatch comfort and their corresponding influencing factors. Therefore, the present study employed a bottom-up grounded theory approach to construct a user-centered model for smartwatch comfort. Through the coding of in-depth interviews with 64 smartwatch users, we discovered that smartwatch comfort encompasses both physiological dimensions (e.g., pressure and foreign body sensation) and psychological dimensions (e.g., perceived intelligence and satisfaction of needs). Furthermore, the features of smartwatches, including physical attributes (e.g., size and material) and functionalities (e.g., interoperability and automation capabilities), directly impact the comfort experience. Additionally, individual and contextual factors can explain variations in the comfort experience of smartwatches. Users with different physiological characteristics (e.g., wrist size and body sensitivity) and psychological needs (e.g., utilitarian or hedonic needs) are influenced differently by the factors of smartwatches that affect their comfort experience. The adaptability of smartwatches across different contexts (including task context, social context, and temporal context) is also a significant influencing factor on comfort. This substantive grounded theory provides crucial guidance for the selection of core variables in future quantitative research and contributes to the development of smarter, more comfortable, and user-centric smartwatches.